In recent years, specifically along with progress of networks capable of readily sending digital system data, image forming methods employing an electrophotographic system have widened the application region from simple copying action to formation of original images and the use as a so-called alternative to printing.
Consequently, in the above technology, it has been desired to form stable images over an extended period. Further, it is desired to extend the replacement cycle of developers as long as possible.
On the other hand, in the image forming method employing the electrophotographic system, in order to form stable images, it has been considered that it is preferable to employ a two component developer composed of a carrier and a toner. The reason is that since the carrier exhibits charge providing capability, it is capable of assuredly providing charge to the toner, and further, since many triboelectric charge providing sites exist, it is possible to realize a rapid charge rise, whereby it is considered to be appropriate for high speed development.
While employing the two component developer, in order to extend its replacement cycle, it is essential to retard adhesion of the toner to the carrier, and many measures to retard toner adhesion have been proposed However, since eventually, it outlives its usefulness, it necessitates replacement of the entire developer.
In order to decrease the above replacements of the developer as least as possible, proposed is an image forming method called a so-called trickle developing system in which the used developer is gradually removed, while the toner and the carrier in an amount equivalent to the removed is replenished (refer, for example, to Patent Documents 1-3).
In the above trickle developing system, the used developer is gradually removed from the interior of the developing device, and carrier in the corresponding amount and toner in the amount used for image formation and in the amount in the developer which has been removed are intermittently or continuously replenished. By employing the above system, since the deteriorated developer (being the carrier) is gradually replaced with fresh one, excessive deterioration of the developer is retarded, whereby it has been known that no replacement of the developer is required over an extended period.
However, in the image forming method of the above trickle developing system, problems result in which during the use over an extended period, degradation and deterioration of image quality such as background density increase or toner scattering, or staining of the interior of the device occurs.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2001-330985
(Patent Document 2) JP-A 2004-287269
(Patent Document 3) JP-A 2007-079578